This invention relates, generally, to non-aqueous electrochemical cells and more particularly to inorganic cells employing lithium metal as the anode which is spirally wound with a separator and cathode or cathode collector. Commercial electrochemical cells, referred to as lithium/SO.sub.2 cells, typically contain lithium anodes and electrolytes comprised of a salt such as LiBr dissolved in a solvent mixture of liquified SO.sub.2 and an organic co-solvent, such as acetonitrile. The cathodes of such cells are usually comprised of a carbon black such as Shawinigan black (acetylene black) formed on an expanded metal substrate.
Discharge of such cells results in the formation of the anode metal dithionite at the carbon cathode surface. In such cells, the SO.sub.2 fluid cathode depolarizer reacts directly with the anode metal cations to form such dithionite product. Since the cathode reaction is a direct one between the anode and fluid cathode depolarizer, the carbon cathode collector only provides a catalytic surface for such reaction.
The spiral wound configuration permits high currents to be drawn; however, this high power output can render cells unsafe in abusive conditions. For example, it is very difficult to manufacture cells having identical capacities. Therefore, when cells are discharged in series it is possible that one cell will exhaust its capacity before the others and is driven into a state of voltage reversal.
It is recognized that spirally wound non-aqueous electrochemical cells having lithium or similar alkali or alkaline earth metal anodes commonly have a lower cycle life than comparable aqueous systems, which employ cadmium or lead negative electrodes. A major cause of the death of such lithium cells is the formation of dendrites which grow from the lithium electrode and make electronic contact with complementary positive electrodes.
There have been various teachings of the problems resulting from dendrite growth between the anode and cathode. One such teaching can be found in U.S. Pat. No. 4,622,277 dated Nov. 11, 1986. In this reference, it is taught to use an exposed inert conductive metal coupled mechanically and electrically to the cathode and a dendrite target comprised of a second segment of exposed inert conductive metal coupled mechanically and electrically to the anode. It is taught when these electrodes are spirally wound, the two pieces of inert metal are oriented such that they face each other and are held in physical isolation by the separator which is interposed therebetween. During voltage reversal abuse, dendrites grow from the first segment of inert metal to the dendrite target, thereby creating a low resistance pathway between the two pieces of inert metal. This situation can occur relatively harmlessly when the cell is fully discharged as the voltage is driven negative since at this time the cell has minimal energy content.
It has been determined that if a dendritic short occurs external to the cell stack, in a fully charged or partially charged cell, the results can be much more severe. Through this dendritic short passes excessive amounts of current, resulting in excessive heat generation, which may result in cell venting.
It is thus an object of the present invention to provide a cell and method for its manufacture which substantially reduces the formation of alkaline metal-based dendritic shorts external to the spirally wound cell stack.
This and further objects will be more readily appreciated when considering the following discussion and appended drawings, wherein